The present invention relates to a processing liquid suitable for use in the silver complex diffusion transfer reversal process.
The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde--The Focal Press--London and New York, (1972).
In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver solvent into soluble silver complex compounds which are allowed to diffuse into an image-receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values with respect to the silver image obtained in the exposed photographic material.
The developing agent or a mixture of developing agents can be present in an alkaline processing solution and/or in the photographic silver halide emulsion layer material. In case the developing agent or a mixture of developing agents is contained in the photographic silver halide emulsion material, the processing solution can be merely an aqueous alkaline solution that initiates and activates the development.
Suitable developing agents for the exposed silver halide are e.g. hydroquinone-type and 1-phenyl-3-pyrazolidone-type developing agents as well as p-monomethylaminophenol.
The silver halide solvent, mostly sodium thiosulphate, may be supplied from the non-light-sensitive image-receiving element as mentioned above, but it is normally at least partly already present in the alkaline processing solution.
The alkaline processing solution usually contains sufficient alkaline substances to bring the pH above 10, e.g. sodium hydroxide, sodium carbonate and a great many other compounds which can raise the pH, for example borax, tertiary sodium phosphate, lithium hydroxide and amines, particularly alkanolamines.
The use of amines and alkanolamines in processing solutions for the silver complex diffusion transfer reversal process is described e.g. in U.S. Pat. Nos. 2,702,244, 4,568,634 and 4,632,896 and GB 2 159 968.
Tertiary alkanolamines having a pKa value higher than 8.5 and their use in the DTR-process are described in Research Disclosure, Jul. 1987, item 27939.
The DTR-process initially intended for office copying purposes has found now wide application in the graphic art field, more particularly in the production of screened prints from continuous tone originals.
For the latter purpose it is particularly important that the processing characteristics remain steady for a large set of prints and that the gradation, optical density (transmission density in the case of film material and reflection density in the case of opaque material e.g. paper material) and the neutrality of the colour tone (black) of the screen dots in the screened prints satisfy graphic art standards which are particularly high compared with normal copying.
It has been established experimentally that DTR-processing solutions containing alkanolamines and more particularly tertiary alkanolamines as alkalinity source offer the advantage of having a comparatively low carbon dioxide absorption and consequently provide a better pH stability and more equal reaction kinetics to the processing solution over a prolonged period of time.
Unfortunately when using alkanolamines DTR-images are obtained that are more or less stained and of which the silver images are not purely black (ref. the above mentioned book of A. Rott and E. Weyde, p. 66).
As is described in the same book on said page 66 it is common practice to avoid the formation of silver images having a brown or yellow image by using silver image-toning agents such as heterocyclic mercapto compounds which influence the amount, the nature and the grain size of the silver deposits of the reduced silver. As a very effective black-toning agent 1-phenyl-1H-tetrazole-5-thiol also known as 1-phenyl-5-mercapto-tetrazole has been used, and such by incorporating it in the alkaline aqueous processing liquid and/or in the image-receiving material. Yet, when using said very effective black toning agent in the circumstances wherein alkanolamines including tertiary alkanolamines having favourably low CO.sub.2 -absorption capacity, are the preponderant alkalinity source, the neutrality of the image tone and reduction of stain in the non-image parts is still to be improved, especially in the lighter shades of the silver image (i.e. in the lower density parts which correspond with the toe of the sensitometric curve) that still show a yellowish colour tone. This is true not only in the case of light-sensitive materials of the negative type producing direct-positive images in the image-receiving material but also and even more pronounced in the case of light-sensitive materials of the direct-positive type producing negative images in the image-receiving material.